1. Field of the Invention
The invention relates to a magnetic head including an at least substantially flat magnetic coil having at least one coil layer structure comprising an electrically conductive winding. The magnetic head is especially meant for use in a magneto-optical device.
2. Description of the Related Art
An embodiment of such a magnetic head is disclosed in International Patent Application No. WO-A 98/48418, corresponding to U.S. Pat. Nos. 5,886,959 and 5,903,525. The magnetic head known from this PCT document includes a flat magnetic coil having two parallel coil sections, each coil section comprising a plurality of windings formed by means of a thin-film technology. The magnetic coil extends in a magnetic yoke which, with the coil, defines a central passageway for an optical beam. The known magnetic head is intended for use in a magneto-optical (MO) system for the storage of data in a magneto-optical (MO) medium comprising a recording layer, the magnetic head being situated at a short distance from a main face of the MO medium during the recording or reading of data. Such a magneto-optical system comprises, apart from said magnetic head, a laser source and optical elements, which include a focusing lens, enabling a laser beam to be routed to a recording layer via the central passageway. During the storage of data, the laser beam is utilized for reducing the coercivity of the recording layer of the MO medium, in that selected spots are heated to approximately the Curie temperature of the recording layer. Meanwhile, the magnetic coil is activated to generate a time-varying magnetic field traversing the recording layer in order to define a pattern of magnetic domains. During reading-out of stored information, the MO medium is scanned with the laser beam, use being made of the magneto-optical Kerr effect which is known per se.
During the magneto-optical storage of information, the minimum width of the stored data bits is dictated by the diffraction limit, i.e., the numerical aperture (NA) of the focusing lens used and the wavelength of the laser beam emitted by the laser source.
A reduction of said width is generally based on shorter-wavelength lasers and higher-NA optical focusing systems. During magneto-optical recording, the minimum bit length can be reduced to below the optical diffraction limit by using Laser Pulsed Magnetic Field Modulation (LP-MFM). In such a process, the bit lengths are determined by the pulsing rate of the laser in combination with the alternating magnetic field. For reading the small marks recorded in this way, use is made of magnetic super resolution (MSR) technologies, including Domain Expansion methods, like MAMMOS, based on media with magnetostatically coupled layers. An advantage of Domain Expansion methods is that marks with a length below the diffraction limit can be detected with a similar signal-to-noise ratio, as marks with a size comparable to the diffraction limited spot.
A magnetic amplifying magneto-optical (MO) system, called MAMMOS, is described in the IEEE Journal of Selected Topics in Quantum Electronics, Vol. 4, No. 5, September/October 1998, pages 815-820.
The concept of MAMMOS is to realize a signal enhancement by using magnetic-domain expansion at the readout time. Use is made of a storage medium comprising a storage layer and readout layer magnetostatically coupled to each other. During readout, a laser beam heats the medium at the readout position. When a mark in the storage layer, i.e., a bit with a magnetization direction opposite to the initial magnetization direction of the readout layer, is within an area called the copy window, where the temperature is high enough to enable magnetization reversal, a domain is nucleated in the readout layer. An external magnetic field is applied for expansion of this domain, the field being reversed to collapse the domain after detection of the domain.
The coil used in the magnetic head known from said WO-A 98/48418 has a limited readout resolution when applied in combination with MAMMOS.